The present invention relates to an axial flow fan for moving air through a heat exchanger and is preferably for use in the cooling and heating systems of motor vehicles.
Fans of this type must meet certain requirements, among which: low noise level, high efficiency, compact dimensions and ability to obtain good values of pressure head and delivery.
Patent EP-0 553 598 B in the name of the same Applicant as the present, discloses a fan with blades having equal spacing angles. The blades have a constant chord length along their entire length and they are delimited at the leading and trailing edges by two curves which, when projected onto the plane of rotation of the fan wheel, are two circular arcs.
Although fans made in accordance with this patent achieve good results in terms of efficiency and low sound pressure, the sound distribution of the noise may be irritating to the human ear.
In fact, with the blades spaced at equal angles, there are cases of resonance with a main harmonic whose frequency is the product of the number of revolutions per second of the fan wheel multiplied by the number of blades. This resonance gives rise to a hissing noise which may be irritating to the human ear.
Even if the perception of irritation caused by a sound is mainly subjective, there are basically two reasons which influence the noise disturbance: the degree of sound pressure, that is, the intensity of the noise and how it is distributed in terms of tone. As a result, low intensity noises can also become irritating if the tone distribution of the noise distinguishes it from background noises.
To solve this problem, fans with blades spaced at unequal angles have been made.
Calculating an average of the sound intensity values at various frequencies, with the blades spaced at unequal angles the noise produced is almost equal to that with the blades spaced at equal angles. However, the different tone distribution of the noise allows an improvement in the acoustic comfort. However, the fans with the blades spaced at unequal angles have a number of disadvantages.
The first disadvantage is the fact that in many cases the efficiency of the fans with blades spaced at unequal angles is less than that of the fans with spaced blades of equal angles.
Another disadvantage is the fact that the fan wheel with blades spaced at unequal angles may be unbalanced.
The aim of the present invention is to provide an improved axial fan with a very low noise level.
Another aim of the present invention is to provide an improved axial fan with good efficiency, head and delivery values.
Yet another aim of the present invention is to provide an improved axial fan whose fan wheel is substantially balanced naturally.
In accordance with an aspect of the present invention, an axial fan is disclosed as specified in the independent claim. The dependent claims refer to preferred, advantageous embodiments of the invention.